Known in the art is an internal combustion engine configured arranging in an engine exhaust passage an NOx storage catalyst storing NOx contained in exhaust gas when an air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich, arranging in the engine exhaust passage upstream of the NOx storage catalyst a small-sized fuel reforming catalyst having a cross-section smaller than a cross-section of the exhaust passage to make part of the exhaust gas discharged from the engine flow into the fuel reforming catalyst, and injecting fuel toward an upstream side end face of the fuel reforming catalyst when the NOx storage catalyst should release NOx (for example, see Japanese Patent Publication (A) No. 2005-127257).
In this internal combustion engine, when the NOx storage catalyst should release NOx, the injected fuel is reformed in the fuel reforming catalyst and the reformed fuel, for example, a high reducing ability fuel containing H2 or CO, is sent to the NOx storage catalyst. As a result, the NOx released from the NOx storage catalyst can be reduced well.
However, even if the reformed fuel is sent into the NOx storage catalyst in this way, when no reduction reaction occurs in the NOx storage catalyst such as if the NOx storage catalyst is not activated, the problem arises that the reformed fuel sent to the NOx storage catalyst passes straight through the NOx storage catalyst and is discharged into the atmosphere. To prevent such a problem from occurring, it is necessary to control the feed of fuel to the fuel reforming catalyst to match the objective in accordance with the state of the fuel reforming catalyst or NOx storage catalyst.